Astrobiology

Determining if life exists elsewhere in the universe other than on Earth is one of the key missions of the Space Science Enterprise areas of NASA. The paper published by McKay et al. (1996) has stimulated great interest in the search for life on Mars and elsewhere. An important issue to consider in the search for extinct or extant life on Mars (and other planets) is the choice of biomarkers (e.g., Gibson et al. 2001). These biomarkers must play a major role in the biochemistry of any known organism on Earth if life on Mars and other planets are carbon-based aqueous chemistry. Amino acids were used as biomarkers (Bada et al., 1998). However, detection of amino acids is not conclusive, because (1) amino acids are soluble in water solutions and thus amino acids can be replaced in the meteorite stone after its arrival, by water solutions that contain amino acids on Earth; (2) amino acids can be produced abiotically and by recemerization of isomers. Phospholipids are the biomarkers to search for when trying to determine whether life ever originated and proliferated on Mars. Phospholipids are major components of cell membranes of all organisms and form lipid bilayers in biomembranes. Thus, because there is no direct evidence that a living entity evolved or exists on Mars and membrane is a basic characteristic of all life, analysis for lipids from Martian meteorites would provide an unambiguous way of detecting extinct or extant life on Mars.

Lipids are a primary component of bacterial cell membrane in which important biological functions occur (metabolism, reproduction, etc.). A unique feature of archaea is their ether-linked membrane lipids, which are derived from C20-isopranyl diether and its tetraether dimers (isopranyl glycerol di- and tetraethers) (De Rosa and Gambacorta, 1988; ). Archaeal core lipids are the only known source of these lipids. These lipids are readily analyzed by gas chromatography (GC/MS; e.g., Fang and Findlay, 1996; Hinrichs et al., 1999) and liquid chromatography/mass spectrometry (LC/MS; Fang and Barcelona, 1998; Hopmans et al., 2000). Bacteria contain ester-linked lipids: glycolipids and phospholipids (White et al., 1979) which can be readily distinguished from ether-linked archaeal lipids. Both ether-linked and ester-linked lipids can be used to determine microbial biomass and community structure (White, 1988; Nichols et al., 1987).

A project funded by NASA Johnson Space Center entitled "Lipid Biomarkers as Indicators of Extant or Extinct Life on Mars (and Other Planets)" is ongoing.

References:
Gibson, E.K., McKay, D.S., Thomas-Keprta, K.L., Wentworth, S.J., Westall, F., Steele, A., Romanek, C.S., Bell, M.S., Toporski, J., 2001. Precambrian Res. 106, 15-34.

McKay, D.S., Gibson, E.K., Thomas-Keprta, K.L., Vali, H., Romnek, C.S., Clemett, S.J., Chillier, X.D.F., Maechling, C.R., Zare, R.N., 1996. Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH 84001. Science 274, 923-930.

De Rosa, M., Gambacorta, A., and Gliozzi, A., 1986. Structure, biosynthesis, and physicochemical properties of archaebacterial lipids. Microbiol. Rev. 50, 70-80.

Fang, J. and Findlay, R.H., 1996. The use of a classic lipid extraction method for simultaneous recovery of organic pollutants and microbial lipids from sediments. J. Microbiol. Methods, 27, 63-71.

Fang, J. and Barcelona, M. J., 1998. Structural determination and quantitative analysis of bacterial phospholipids using liquid chromatography/ electrospray ionization/mass spectrometry. J. Microbiol. Methods 33, 23-35.

Fang, J., Barcelona, M.J., Nogi, Y. and Kato, C., 2000. Biochemical implications and Geochemical significance of novel phospholipids of the extremely barophilic bacteria from the Mariana Trench at 11,000 m. Deep-Sea Res. I, 47, 1173-1182.

Hopmans, E. C., Schouten, S. Pancost, R. D., van der Meer, M. T. J., and Sinninghe Mamste, J. S., 1987. Analysis of ontact tetraether lipids in archaeal cell material and sediments by high performance liquid chromatography/atmospheric pressure chemical ionization mass spectrometry. Rapid Communications in Mass Spectrometry. 14, 585-589.

Nichols, P. D., Mancuso, C. A., and White, D. C., 1987. Measurement of methanotroph and methanogen signature phospholipids for use in assessment of biomass and community structure in model systems. Org. Geochem. 6, 451-461.

Nichols, D., Bowman, J., Sanderson, K., Nichols, C. M., Lewis, T., McMeekin, T., and Nichols, P. D., 1999. Developments with Antarctic microorganisms: culture collections, bioactivity screening, taxonomy, PUFA production and cold-adapted enzymes. Curr. Opinion Biotechnol. 10, 240-246.

White, D. C., 1988. Validation of quantitative analysis for microbial biomass, community structure, and metabolic activity. Arch. Hydrobiol. Beih. 31, 1-18.

Fig. 1 A typical structure (A) and an example (B) of phospholipid.